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   难度：Hard
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   <h1 class="question_title">
    233. Number of Digit One
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    Given an integer n, count the total number of digit 1 appearing in all non-negative integers less than or equal to n.
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     Example:
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<strong>Input:</strong> 13
<strong>Output:</strong> 6 
<strong>Explanation: </strong>Digit 1 occurred in the following numbers: 1, 10, 11, 12, 13.
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    233. 数字1的个数
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    给定一个整数 n，计算所有小于等于 n 的非负整数中数字 1 出现的个数。
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    <strong>
     示例:
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   <pre><strong>输入:</strong> 13
<strong>输出:</strong> 6 
<strong>解释: </strong>数字 1 出现在以下数字中: 1, 10, 11, 12, 13 。</pre>
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